18 Watt Lite HUM when on standby!

[big_teee]

Thanks for the explanation.
On the hot transformer, the heat is usually from the 6.3V Heater current.
The 120ma will be the HT.
You could have a 120ma PT that runs just as hot as a 100ma PT, if the Heater current is rated the same.
My 50 watt amps will heat up some just in standby.
That is heater winding current.
GL,
T

[rockybottom16]

I tried grounding the speaker frame, which produced no change. So I removed the OT bolt (athe one that wasn't hidden by the board) and spun the transformer slowly 180 degrees while the chassis was on my bench, powered up with speaker connected. As soon as I began to rotate it the hum started to subside a little. So now the laminates are still at 90 degrees to the PT, but the tranny is about 4" further away. It still has some hum, but only about half of what it was; and I can live with it now. The best part is the OT is near the edge of the chassis, so I could install the new bolt without taking the board out. The speaker being used is G12H30, and their efficiency and low-end grunt tend to make that low hum more noticable than most speakers anyway. When I hook up an Eminence Private Jack, you can barely hear any hum now.

Man, these amps are pretty loud. And anything past 3 or 4 on the dial is crunch heaven!

Thanks for all your help, guys.

[Chuck H]

Short of redesigning, which seems impractical now that the hum is tolerable, thee is one other trick that may help. I've never applied it to your problem but I can't see why it wouldn't work...

Solder a 6" lead wire to one of your preamp filaments. Cap the unconnected end with shrink tube. Then move it around near preamp leads. You'll find that in most places it induces some audible hum, but in some places it may reduce hum. The idea is to induce the right amount of hum in the right phase in the preamp so that it's equal in amplitude and out of phase with the hum in the OT. I'm not 100% that this can work with hum inducted to the OT by proximity, but it may and it's easy to try it.

[Enzo]

Hey, if it works, you are a genius, and if it doesn't work, remove the wire and get on with life.